The present invention relates to dot matrix printers, and more specifically, to a method and apparatus for providing a variable printing speed in such printers.
Typically, dot matrix printers, and ink jet printers in particular, have a nozzle, or printhead, which projects ink drops, or dots, towards a recording medium. The dots are deflected vertically in parallel and adjacent scans while the print nozzle moves horizontally across the recording medium, usually paper, to print a line of characters. Each scan consists of a vertical column of matrix, or potential dot, positions. The printer completes each scan, or column of potential dot positions, before moving to the next scan. Where the printhead moves horizontally at a constant speed during each scan, it is well known to those skilled in the art that a skew will be introduced into the characters in the direction of printhead motion unless a means for compensating for the skew is provided. One such skew compensation means is shown in U.S. Pat. No. 3,338,163 which is hereby incorporated by references.
It is desirable in a dot matrix printer to have a slower speed, higher quality print capability and a faster speed, lower quality print capability. One prior art attempt at providing this multispeed capability is to double the print speed while deleting alternate scans. This method, however, has produced a noticeable slant in the characters even where a skew compensation means has been employed in that while printhead speed is doubled, the time required to complete each scan and the amount of skew compensation has remained unchanged. Moreover, the slant is always in the direction of printhead motion so that if a first line of characters is printed from left to right, the characters will noticeably slant to the right; then if the second line of characters is printed from right to left on the return motion of the printhead, the characters will noticeably slant to the left. Printing in this manner is so disturbing to the eye that the method is limited to unidirectional printing. Consequently, the time spent on the return travel of the printhead once it reaches the right side border cannot be utilized for printing and is lost, reducing overall print speed. Increasing print speed, of course, being the very object of this method.
Other prior art in the dot matrix printing art is shown in U.S. Pat. Nos. 4,172,417; 4,115,788; 4,115,787; 4,045,770; 4,025,925; 3,972,052; and 3,938,163.